Influence of Climate and Lithology on Soil Phosphorus

Climate and lithology are master variables of pedogenesis. We hypothesize that differences in parent material composition will influence the outcome of soil P fractionation, in concert with climate and the relative degree of chemical weathering. Here, we investigate a novel climo-lithosequence to elucidate the influence of lithology and climate on P dynamics. Three climosequences (elevational transects) spanning four climatic zones (Blue-Oak, Ponderosa Pine, White fir and Red fir), and three bedrock lithologies (basalt, andesite and granodiorite) were investigated across the Sierra Nevada and southern Cascades. Replicate soil samples were collected by genetic horizon at twelve sites (4 climate zones x 3 lithologies) and characterized by a modified Hedley P fractionation method to quantify P into operationally defined pools. Initial results from the fractionation of andesite and basalt transects (granodiorite forthcoming) show large climatic and lithologic effects on soil P fractions, suggesting that the distribution of soil P and the trajectory of P transformations are significantly influenced by lithology as well as climate. For example, in the climatic zone of least weathering (Red fir), all soil P fractions showed significant lithologic effects. In contrast, with increased weathering, parent material effects on soil P fractions become progressively muted, so that in the zone of most intense weathering (Ponderosa Pine), soil P fractions such as Ca-Pi (1 M HCl-Pi) and labile-Pi (Resin Pi + NaHCO3-Pi), no longer show an influence from lithology. Additionally, significant climatic effects were noted for labile-Pi, Ca-Pi and Fe/Al-Pi (0.1 M NaOH-Pi). A strong positive correlation was observed between poorly crystalline Fe/Al-(hydr)oxides (oxalate extractable Fe and Al) and Fe/Al-Pi (p<0.0001). Conversely, a strong negative correlation was observed between crystalline Fe-oxides (inferred by citrate-dithionite extractable Fe) and Fe/Al-Pi (p<0.0001). Results suggest that P dynamics in soils are strongly influenced not only by climate and the relative degree of chemical weathering, but also lithology, especially during the early stages of pedogenesis. Therefore, parent material and climate may interact more strongly than previously thought to regulate P biogeochemistry.